If you've ever used the API reloadRootControllersWithNames(names: [AnyObject], contexts: [AnyObject]?) in any non-trivial way, you're likely to encounter the frustration of getting this error spamming your logs and causing a lot of issues:

*********** ERROR -[SPRemoteInterface _interfaceControllerClientIDForControllerID:] clientIdentifier for interfaceControllerID:114D0003 not found

Generally this doesn't seem all too helpful. What is interfaceControllerID:114D0003 anyway?

It turns out this is the ID in a private property called _viewControllerID. There is a little bit of code you can add to your willActivate() to see what the _viewControllerID is for each of your pages.

With a little bit of key-value coding trickery, we can get access to the private variable. Knowing exactly which WKInterfaceController is misbehaving really saves time when you're hunting down to clear out all references to the old controller before blowing it away with the reloadRootControllersWithNames call.

The Taptic Engine is a game-changing piece of hardware that Apple has added to the Apple Watch. One of the annoyances of Android Wear and Pebble is that the notification vibrations are not very subtle, and everyone nearby knows that your watch demands attention. The Taptic Engine allows you to send taps to your users wrist, feeling much like someone tapping their finger on you, as a subtle and discreet to communication information for users that they can choose to ignore or receive without bothering those around them.

So what the heck is a Haptic and how is it different from a Taptic?

Haptic Feedback used in technology typically refers to any kind of vibration or force used in input devices on the user. Pressing a button on a smooth glass screen just doesn't feel as satisfying as clicking a physical button on a mouse. That's because the mouse button depresses and then gives a 'click' feeling vibrating into your finger to indicate the button has been pressed. What's nice about this is that you can know when a mouse button has been successfully pressed without looking at it.

Compare this to a button on an iOS App, where you have to visually look where you are aiming your finger, and rely on visual feedback that the button has been pressed correctly. Haptic feedback refers to the class of technology to simulate physical feedback in a device, like the vibrations of a Playstation Controller or the taps on an Apple Watch.

The Taptic Engine is Apple's marketing name for the piece of hardware that can produce Haptic feedback by way of the feeling of taps on your wrist in response to different actions in the hardware. It can notify you that you scrolled to the end of a list, it can make it feel clicky as you click through the items of a WKInterfacePicker list, or even indicate on your driving directions if it's time to turn left or right with different feeling taps so you don't even have to look at the Watch screen to know which way to turn.

Coding Your Own Haptics

The API is exceedingly simple, with just a quick API call to playHaptic():

Apple currently doesn't allow for defining custom Haptic events, so whatever you want to do on your app, you'll have to do it with one of these nine Haptic types.

It's pretty important to understand the feeling, sound and intention of each of these types, so we'll cover each in detail.

WKHapticType in Detail

.Notification

Sound: Chime Haptic: Tap-Tap-Vibrate

The .Notification type is intended for drawing the user's attention when something significant or out of the ordinary has occurred.

.DirectionUp

Sound: Increasing Pitch Haptic: Tap-Tap

The .DirectionUp type is intended for indicating a significant increase value threshold has been crossed such as when moving up a list.

.DirectionDown

Sound: Decreasing Pitch Haptic: Tap-Tap

The .DirectionDown type is intended for indicating a significant decrease value threshold has been crossed, such as when moving down a list.

.Success

Sound: Confirmation Ding Haptic: Tap-Tap-Tap

The .Success type is intended as a confirmation tone to indicate some action has been completed successfully.

.Failure

Sound: Failure Ding Haptic: Long Vibrate

The .Failure type is intended as a failure tone to indicate some action has not been completed successfully.

.Retry

Sound: Quick Ding-Ding-Ding Haptic: Long Vibrate

The .Retry type is intended as a gentle tone to indicate some action has not been completed successfully but the user has an opportunity to retry. Typically you should display some UI with this tone giving the user an opportunity to retry their failed action.

.Start

Sound: Long Ding Haptic: Long Tap

The .Start type is intended to indicate the start of an activity, such when a timer begins.

.End

Sound: Long Ding-Long Ding Haptic: Long Tap-Long Tap

The .End type is intended to indicate the end of an activity, such when a timer has ended.

.Click

Sound: Very soft click Haptic: Light tap

The .Click type is intended to indicate a clicking sound, like when a dial is clicking.

Haptic Demo sample provided on GitHub

Taptic Engine Considerations

The Taptic Engine cannot overlap Haptics, and there is a delay between each one that can be played. So for example, let's say you wanted to play a .Click as you move through each item in a list. If you move quickly through the list, a lot of .Click haptics will be ignored, and your user will find it not feeling quite right when the taps fall out of sync from your UI.

Make sure that when you are designing your application, only use haptics on rare and significant events to both conserve battery as well as feel strange if they are asked to overlap and you end up losing feedback commands that you intended.

Trying It Out

We've provided sample code for an app to try out each of the haptic types on the Sneaky Crab GitHub. Unfortunately, the current version of the watchOS Simulator does not support sounds, so you'll have to install this on your iOS 9/watchOS 2 hardware to try it out.

Remember that it is currently impossible to reverse a watchOS 2 update, so make sure that you only update to watchOS 2 for test devices and not your main Apple Watch, especially considering that watchOS 2 beta 1 is extremely unstable!

Wrapping Up

Apple provides a very simple API for providing haptic feedback to a user through the excellent Taptic Engine. Staying consistent with the intended meanings of the different types will allow a user to learn and understand what you are trying to convey without having to relearn anything while using your application.

Apple has provided a new WKInterfaceObject called WKInterfacePicker. It allows you to select an item from a list, similar to the UIPickerView you're already familiar with in iOS 8. You can use it on a list of text, images, and the items are selected by using the Digital Crown.

Picker Styles

There are three main styles you can choose from: List, Stack and Sequence.

The List style is very similar the the UIPickerView in iOS, where you use the crown to select a sequential list of items. The items can be images and text, just text or just images.

The Stack Style animates the items like a stack of cards, providing an attractive way to select an item.

The Sequence Style replaces each image, allowing you to quickly flip between your choices, and also can be used to animate progress bars.

Focus Styles

For each style, you can indicate a focus style, which indicates how you want watchOS to display which picker is in focus. If there is only one picker on the screen, you may want to use None. If you have multiple pickers, you need to indicate which control is in focus and will be affected by the Digital Crown. For example, when customizing the clock face, Outline focus styles are used to indicate which complication is in focus to be adjusted by the Digital Crown.

Each picker item can be annotated by a caption, which can be used to indicate the group or secondary information about the item that will be displayed while picking. The caption can be useful if you are picking images, for example.

Focus Style: None

Focus Style: Outline

Focus Style: Outline With Caption

Coding

Open the Storyboard for your WatchKit App, and drag a Picker into your InterfaceController.

Choose the Style and Focus Style. The Indicator allows you to toggle whether the scroll guide appears beside the Digital Crown. This can be useful for styles like Stack, where it may not be clear to the user where in the list they have scrolled to.

Connect your picker to your interface controller twice, one for the outlet and one for the action.